Catalytic reforming, or hydroforming, is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multi-functional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, such as alumina. The alumina support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and hydrocracking reactions.
Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semi-regenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semi-regenerative and cyclic reforming amd moving-bed reactors in continuous reforming. In semi-regenerative reforming, the entire reforming process is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in series. In continuous reforming, the reactors are moving-bed reactors as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst.
All of the reforming catalysts in general use today contain platinum supported on an alumina or an alumina-silica base. In many cases, rhenium is combined with platinum to form a more stable catalyst which permits operation at lower pressures. As previously mentioned, platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation reactions and halogenated alumina provides an acid site for isomerization, cyclization, and hydrocracking reactions. Halide, particularly chloride, is known to be a catalyst promoter when added to a reforming catalyst in catalyst-promoter quantities since larger amounts can cause excessive undesirable hydrocracking reactions. While most feeds contain small amounts of halide, it is usually not enough to adequately maintain catalyst activity. Consequently, platinum-containing alumina-based reforming catalysts are manufactured having a predetermined amount of halide, particularly chloride, on catalyst, sometimes up to about 3 wt. %, depending on the active metals content of the catalyst. As the catalyst ages, chloride loss becomes appreciable and, inter alia, contributes to loss of catalyst activity. Various approaches have been taken to address the need for maintaining desirable levels of halide on catalyst, as well as preventing its loss when onstream. For example, U.S. Pat. No. 2,899,378 teaches a reforming process wherein dry halide, in the form of a halogen, halide acid, or an alkyl halide, is injected into one or more series reactors to maintain the halide concentration within the desired range.
Also, U.S. Pat. No. 3,287,253 teaches a staged reforming process in which a platinum-alumina catalyst used in a third reactor is chlorinated by introducing a chloride-containing compound upstream thereof.
Further, U.S. Pat. No. 3,381,048 discloses the isomerization of xylene isomers over a regenerable platinum-alumina-halogen catalyst in which the chloride is maintained at a certain level by addition of a chloride-containing substance to the feed, or by treating the catalyst directly with a chloride-containing substance.
It is suggested in U.S. Pat. No. 3,573,199 that the acidity, or activity, of a reforming catalyst can be controlled by using a selected temperature range and water content. This allegedly allows a more uniform distribution of halide throughout the catalyst. It is also suggested in this reference that for several reactors in series, proper allocation of halide in each reactor can be obtained. It is to be noted, however, that in the process of this reference, halide addition is discontinuous, the temperature level and water concentration is adjusted prior to halide injection, and the halide concentration on catalyst, in fact, increases in downstream reactors.
U.S. Pat. No. 3,649,524 teaches on-stream chlorination wherein water is injected upstream of a first reactor to maintain a water level throughout the series of reactors at 5 to 25 vppm, and chloride is injected upstream of each reactor to provide a chloride to water molar ratio of about 0.05 to about 1. Such an approach results in overchlorination of the catalyst, thus contributing to elevated acid cracking and loss of activity and selectivity.
While these various attempts have been made to optimize the use of halide as a promoter for reforming catalyst, there still exists a need in the art for still improved methods for maintaining a predetermined optimum level of halide throughout a series of reactors of a reforming process unit.